That class x stuff is about the max transfer speeds supported, no? Don't the higher class ones always support slower speeds too, possibly even all slower speed settings? I've not heard before of a device that can't read a particular card because it is too high a class. (Any issues tend to be about the storage capacity and the file system needed to fill such a size.) As far as I know you just don't benefit from the extra speed, as the effective speed is determined by the slowest of the two devices (card and reader) involved.

I tried one, it works. There are no speed benefits however. It actually seems slower, but that may also be related to the capacity (32gb) rather than the speed rating/controller. (I don't have it on hand right now to give you the exact make & model)

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Now I checked the speed with a benchmark program and attached the result image, which says that the reading speed is about 4mb/s and the writing speed is about 2mb/s.
So I decided to buy SD card reader for it. I assume moving SD card seems easier than moving USB cable to copy something from PC to SD card of DR1000S, for me.